The present invention relates to gearing for high-speed transmission and, more particularly, to high-speed gears having two toothed portions juxtadisposed on a shaft for use in applications wherein the absorption of dynamic loads, the elimination of backlash and the minimizing of noise and vibration are critical.
In certain high-speed applications, for example in turbomachinery, gears may be subjected to high dynamic loads. Unavoidable inaccuracies in the teeth mesh due to tolerance, as well as errors in manufacturing and assembly, lead to high-frequency periodical accelerations of the driven gear which result in the imposition of acceleration forces on the meshing teeth. Due to the presence of backlash between non-working teeth flanks, the acceleration of the driven gear leads also to separation of the working teeth followed by a reengagement thereof resulting in an impact load being imposed on the gear teeth. This phenomenon, known as free impact or hammering, results in high dynamic loading on the gear teeth with attendant noise generation and vibrations occurring. Additionally, in symmetrical double helical or herringbone type gears, the teeth meshing errors cause the uneven distribution of applied load between the two toothed portions of the gear. Accordingly, efforts have been made when designing double toothed helical gears to eliminate or at least to reduce these disadvantages.
For example, in U.S. Pat. No. 2,982,146 there is disclosed a gear mechanism wherein one gear is axially fixed while the other gear is free move axially along its shaft or axially together with its shaft. Each gear is comprised of two assymmetric toothed portions with the teeth on one toothed portion having a helix angle of less than 45.degree. pitched in one direction while the teeth on the other toothed portion has a helix angle of more than 45.degree. pitched in the opposite direction. The free floating gear serves to distribute a transmitted load between the two toothed portions of the gear in inverse proportion to the relationship between the tangents of the two helix angles, the load on both toothed portions of each gear being applied in the same direction. Because the substantially greater part of applied load is taken-up permanently by the toothed portion having the lesser helix angle, this gearing is less sensitive to the variations of load-distribution due to mesh errors than is gearing of the symmetrical herringbone type. However, this gearing is still subject to hammering due to the absence of backlash take-up capability.
An example of anti-backlash gearing is presented in U.S. Pat. No. 3,648,534. As disclosed therein, one of the meshing gears is split gear having two toothed portions which are not integral but rather are angularly adjustable. One of the toothed portions of the split gear is fixed to the shaft while the other to free to rotate about the shaft under the influence of a biasing means which controls the angular displacement between the two portions of the split gear. The biasing means imposes an additional load on the gear teeth which permanently acts on the teeth and can lead to undue wear and gear loadings particularly in high speed applications.
U.S. Pat. No. 4,036,074 discloses anti-backlash gearing of the split gear type wherein the split gear has two toothed portions whose teeth have the same helix angle and pitch direction. One portion is fixed to the shaft while the other portion is axially movable along the shaft and separated therefrom by spring biasing means. The spring means serve to urge the movable toothed portion of the split gear away from the fixed toothed portion of the split gear. The maximum displacement of the movable portion away from the fixed portion is preadjusted to a preselected distance to limit backlash. Although, this gear arrangement subject to reduced backlash, backlash still exists and, in fact, actually varies from one pair of engaging teeth to the next pair of engaging teeth.
It is the primary object of the present invention to provide a gear assembly having meshing gears of the double toothed portion type wherein the free impact (hammering) of teeth is eliminated without imposing additional loading on the teeth.